In the case where an NOx storage reduction catalyst (which will be simply referred to as the NOx catalyst hereinafter) is provided in an exhaust passage of an internal combustion engine, NOx stored in the NOx catalyst may be reduced by decreasing the air-fuel ratio of the ambient atmosphere around the NOx catalyst to a target air fuel ratio. Known ways of decreasing the air-fuel ratio of the ambient atmosphere around the NOx catalyst include adding reducing agent to the exhaust gas through a reducing agent addition unit provided in the exhaust passage upstream of the NOx catalyst, and decreasing the air-fuel ratio of the exhaust gas discharged from the internal combustion engine.
Patent document 1 discloses a technology in which when NOx stored in an NOx catalyst is to be reduced, the air-fuel ratio of the exhaust gas is decreased by regulating at least one of the EGR quantity or the intake air quantity, and thereafter the air-fuel ratio of the exhaust gas is further decreased by adding fuel to the exhaust gas.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-226463